Activation of cardiac muscle is dependent on Ca2+-binding to cardiac troponin releasing tropomyosin from an inhibitory position on the thin filament. It is well established that Ca2+-sensitivity of force decreases as the sarcomere length shortens and vice versa. It is this length-dependent shift in force ? Ca2+-sensitivity provides the main basis for the steep relation between the filling pressure at the end of diastole and the myocardial force generation in the intact heart (the Frank-Starling law of the heart). As the sarcomere length decreases, the separation between the contractile filaments (lattice spacing) increases. It has been proposed that the spreading of the filaments (increasing lattice spacing) could decrease the probability of the myosin (cross-bridge) binding to actin and hence force generation. To determine the relationship between lattice spacing and cross-bridge binding to actin requires high-resolution in vivo measurements. X-ray diffraction is one of the few techniques that can simultaneously determine distances between the filaments and the fraction of cross-bridges bound to actin in a living muscle cell. Our goal is to apply the X-ray diffraction technique to study the mechanism of the Frank-Starling relationship. Low angle X-ray diffraction measurements of myofilament lattice spacing (D1,0) and equatorial reflection intensity ratio (I1,1/I1,0), which measures the fraction of myosin bound to actin, were made in relaxed skinned cardiac trabeculae from rats. We first tested the hypothesis that the degree of ?weak? cross-bridge binding, which has been shown to be in the pathway for force generation in skeletal muscle, is modulated by changes in lattice spacing in permeabilized cardiac muscle. Altered weak cross-bridge binding was detected not only by changes in I1,1/I1,0 but also by measurements of chord stiffness (chord K). Both measurements showed that, similar to skeletal muscle, the probability of weak cross-bridge binding was significantly enhanced by lowering the temperature. In addition, changes in D1,0, I1,1/I1,0 and chord K by osmotic compression with Dextran T500 were determined at various sarcomere lengths. At each sarcomere length increasing [Dextran] caused D1,0 to decrease and both I1,1/I1,0 and chord K to increase, indicating increased weak cross-bridge binding. The results suggest that in intact cardiac muscle increasing sarcomere length and decreasing lattice spacing could lead to increased force by increasing the probability of initial weak cross-bridge binding. Further studies of the filament structures are in progress.